1. Field of the Invention
This invention describes aminoplast compositions, methods for curing such compositions, catalysts useful in curing the compositions and articles made from the cured aminoplast resin.
2. Introduction to the Invention
This invention describes, in particular, catalysts which are useful for curing aminoplast compositions. A generalized definition of an aminoplast resin as may be used herein is a class of thermosetting resins made by the reaction of an amino compound with an aldehyde to give a substantially sulfur-free resin product. Typically, the aldehyde will be formaldehyde and the most important amino compounds being urea and melamine. These materials and their methods of preparation will be discussed further in the specification.
The aminoplast resins are useful in such diverse areas as molding, adhesives, laminating, textile finishes, permanent-press fabrics, wash and wear apparel fabrics, protective coatings, paper manufacture, leather treatment, binders for fabric, foundry sands, graphite resistors, plaster of paris fortification, foam structures and ion-exchange resins.
As with most chemical compositions, the aminoplast resins may be enhanced in their curing rate through the use of a catalyst. The use of a catalyst is particularly important when the composition is flowable or is a molded composition. In such cases, it is desired that the aminoplast resin be cured quickly to avoid sagging, in the case of a flowable resin, and rapidly when a potted composition is prepared so that the mold may be reused.
A particularly important feature in the curing of an aminoplast resin, particularly in coatings and paints, is that the catalyst employed should not be particularly water-soluble. When the catalyst system is water-soluble, such as when para-toluene sulfonic acid is employed, the coating can deteriorate due to the affinity of the catalyst for water. A further important feature in the selection of a catalyst for an aminoplast coating or paint composition is that the catalyst be of sufficient strength to allow rapid curing at low temperatures. A particularly advantageous aspect of the present invention is low temperature curing with high water tolerance, e.g., the coating lasts longer.
It is known from Gloyer et al in U.S. Pat. No. 3,287,318 issued Nov. 22, 1966 that various metal phosphate salts may be employed in the curing of aldehyde-substituted amide interpolymers. U.S. Pat. No. 4,352,842 to Kooymans et al describes the use of zinc short chain fatty acid salts for the curing of epoxy-derived resins.
U.S. Pat. No. 3,743,612 issued July 3, 1973 to Vial describes zinc ammonium salts for the curing of carboxylated resins. Vial discloses the presence of copolymers in his carboxylated latex as including melamine-formaldehyde condensates, urea, urea-formaldehyde condensates, methylated melamine-formaldehyde condensates, methylated urea-formaldehyde condensates, butylated melamine-formaldehyde condensates, butylated urea-formaldehyde condensates and phenol-formaldehyde condensates.
Noyes, in U.S. Pat. No. 3,992,338, issued Nov. 16, 1976 states that phosphate esters may be employed as curing agents for resins. In particular, Noyes states that small quantities of phosphoric acid may accelerate the cross-linking of the resins employed in this patent. Noyes describes nitrogen containing resins as useful in his composition including those made from an aldehyde and urea, N,N'-ethylene urea, dicyandiamine and aminotriazines, such as melamines and guanamines. It is observed that the phosphoric acid suggested by Noyes for curing various resin systems is a highly water-soluble material.
Chang et al, in U.S. Pat. No. 4,297,448, issued Oct. 27, 1981, describe the use of toluene sulfonic acid or phosphoric esters as being useful for curing thermosetting coating compositions. U.S. Pat. No. 4,371,657 to Chang which issued on Feb. 1, 1983 describes various aminoplast resins which may be employed in coating compositions for the exteriors of trucks and automobiles.
U.S. Pat. No. 4,330,458 issued May 18, 1982 to Spinelli et al describes alkylated melamine formaldehyde cross-linking agents which are cured through the use of an acid catalyst. The compositions of Spinelli et al are stated to be particularly useful as an exterior finish for automobiles, trucks, airplanes and appliances, as well as coil coatings.
Chattha, in U.S. Pat. No. 4,323,660, issued Apr. 6, 1982 describes amine aldehyde cross-linking agents and the reaction product of an epoxy functional polymer containing at least two hydroxyl groups per molecule and a sulfonic acid having a branched or linear alkyl or aryl group thereon.
U.S. Pat. No. 4,374,164 to Blank, issued February 15, 1983 discloses that high solid polymer resin coating compositions cross-linked with an alkylated melamine resin may be obtained. The Blank patent discloses numerous melamine resins which may be used in forming coating compositions.
U.S. Pat. No. 4,529,665 to Das et al discloses ammonium salts of sulfocarboxylic acids for curing active hydrogen-containing resins. Quinn, in U.S. Pat. No. 4,591,612, describes compositions which may be utilized for coatings which contain thixotropic agents.
U.S. Pat. No. 4,246,376, issued Jan. 20, 1981 to Didomenico, Jr., discloses methylolamino compositions which may be cured with high molecular weight fatty diols or higher polyols. U.S. Pat. No. 3,674,701, issued July 4, 1972 to Yochum et al, describes accelerator compositions comprising a thiourea, a poly-sulfide and a tetramethylthiuram disulfide compound and a salt of the formula ((RO).sub.2 P(S)S).sub.2 Zn or a zinc salt of the reaction product of ethanolamine, formaldehyde and carbon disulfide.
Various melamine structures and the reactions with polyols are disclosed in Techniques to Measure Melamine/Polyol Reactions in a Film, Lazzara J. Coatings Technology, Vol. 56, No. 710, March, 1984.
Further suggested sources for review concerning aminoplast coatings include a paper entitled Effect of Catalyst Structure on the Properties of Coatings Cross-linked with Hexa(methoxymethyl)melamine, by Calbo, Journal of Coatings Technology, Vol 52, No. 660, January, 1980, pp 75-83. The reader is also directed to Cross-linking Chemistry and Network Structure in Organic Coatings, J. Polymer Science, Vol. 18, 1997-2014 (1980) by Bauer et al. Bauer further reports his work on coatings in the same journal at Volume 18, 2015-2025.
Calbo has further reported information on coatings in Catalyst Selection for High Solids Coatings, Calbo, High Solids Coatings, September, 1982, page 15 et seq. Bauer has further reported his work in network formation and degradation in urethane and melamine-formaldehyde cross-linked coatings in a paper presented at the American Chemical Society spring meeting in 1987 at Denver, Colo. Baker, in February, 1985, presented a paper to the Water-Borne & Higher-Solids Coatings Symposium on the subject of acid catalysts for melamine resins.
It has now been found that through the use of certain sulfur and phosphorus containing molecules that the catalyzed curing of aminoplast resins may be accomplished at low temperatures and while imparting a high degree of water resistance of the cured coating. The aminoplast resins, as later described herein, are cured to a hard, and water stable coating through the use of the technology employed herein.
Throughout the specification and claims, percentages and ratios are by weight, temperatures are in degrees Celsius and pressures are in KPa gauge unless otherwise indicated. Ranges, weights and ratios given herein are exemplary and may be combined if desired. To the extent that references cited herein are applicable to the present invention, they are herein incorporated by reference.